Hi there I am about to put my commander together and can't decide which way to go. My army is made of lots of fire warriors , flame crisis, a riptide, cadre fire blade, ethereal, longstrike and 3 more hammerheads. My thought was to use a coldstar as a quick reaction model to plug any gaps etc. what way should I go?

Both, really.Both are good and both have their place in an army. It just depends what you are lacking. You are lacking both, mobility and anti-tank with the units you listed but Longstrike + Railgun Hammerheads can do okay-ish for anti-tank duty I guess.

Just make sure to magnetize your regular Commander so you can switchout weapons&support systems depending on what you need/want. Unless you play a heavily narrative army your list won't be the same everytime you play.

Magnets are super easy, try not to let it intimidate you too much. Lot of good tutorials floating around on the internet (unfortunately most of the great ones on here got eaten by Photobucket, but I think a few still work).

Even though using magnets sound scary, they're actually not that hard to use. I would very much strongly suggest you just magnetize your commander.

I've been playing since 5th edition and magnets just make life so much easier. Especially since there is an extremely high chance GW will nerf the 4 FB commander in our actual codex release since it is so dominant. You don't want to be stuck with one if it literally can't be fielded anymore (this is what happened with my first commander from years ago it had terrible weapon selection and now just sits on my shelf).

As for which you should field they serve two different purposes. The coldstars is a mobile fire support platform. It can be anywhere at any time and is incredibly safe but not super great at killing stuff. The 4 FB commander is more of a suicide bomb that wrecks one thing and is likely out of position and open to retaliation. Not always, but often.

The correct answer is Cyclic Ion Blaster. But all jokes aside, magnetize them for sure. You can even just magnetize the jet pack pieces and glue the High Output Burst cannon and Missile pod onto the Coldstar jet packs and you choice of weapons on the other. I wouldn't run anything but 4 CIB, 4 Fusion or a Coldstar on a commander.